An inkjet recording system is a printing system in which printing is conducted by spraying a liquid ink with a high degree of fluidity from very fine nozzles, and adhering that ink to a recording medium such as a sheet of paper. These systems enable the printing of high-resolution, high-quality images at high speed and with minimal noise, using a comparatively inexpensive printing apparatus, and are rapidly becoming widespread.
The coloring materials for the inks used in these inkjet recording systems can be broadly classified into materials that use pigments and materials that use dyes. Of these, there is a growing tendency for the use of inks that use pigments as the coloring materials, as such inks exhibit the excellent levels of light resistance, weather resistance and water resistance that are required for high image quality printing.
In terms of the solvent, inks can be broadly classified into aqueous inks and non-aqueous inks. In an aqueous ink, because an aqueous solvent and water act as the ink medium, dispersing a pigment finely within this medium and then maintaining the stability of that dispersion is extremely difficult.
As a result, aqueous pigment inks have been proposed in which the pigment is encapsulated, thereby enabling dispersion within the aqueous medium (see Japanese Patent Laid-Open No. H09-151342 and Japanese Patent Laid-Open No. H11-140343). However, because the inks are water-based, the problem of poor water resistance is undeniable.
In contrast, non-aqueous inks that do not use water as the ink solvent, including solvent-based inks that use a volatile solvent as the main constituent and oil-based inks that use a non-volatile solvent as the main constituent, are now attracting considerable attention. Non-aqueous inks exhibit good drying properties to aqueous inks, and also offer excellent printability.
These non-aqueous inks typically comprise a non-aqueous solvent, a pigment, and a pigment dispersant and the like. Because the commonly used pigment dispersants are polymers, interaction or cross-linking between molecules tends to occur readily within the solvent. These interactions or cross-linking between molecules of the pigment dispersant prevent aggregation of the pigment and enhance the dispersibility, and are therefore necessary to a certain extent. However, if such interactions or cross-linking occur excessively, then if the ink is left sitting in an open system for a long period, normal discharge of the ink from the nozzles of the inkjet discharge head becomes impossible (due to factors such as an increase in the number of non-discharging nozzles or a deterioration in the flight of the ink droplets), and it is now known that these factors can cause white banding within the image or a deterioration in the image precision.
Moreover, particularly in those cases where the printer is left standing in a high-humidity high-temperature environment, it has been found that even if the nozzle portions are subjected to a cleaning operation that includes pushing out and wiping off the ink, some nozzles are not restored (do not return to their normal discharge state).